Numerous applications involve heat-treating a workpiece. For example, in the manufacture of semiconductor chips such as microprocessors, the workpiece typically includes a semiconductor wafer, supported in a thermal processing chamber for annealing or other heat-treating purposes. Commonly owned U.S. Pat. No. 7,501,607, which is hereby incorporated herein by reference, discusses examples of heat-treating techniques for annealing such semiconductor wafers, in which the wafer is first pre-heated to an intermediate temperature, following which the top or device side surface is rapidly heated to an annealing temperature. The initial pre-heating stage occurs at a rate significantly slower than a thermal conduction time through the wafer, and may be achieved by irradiating the back-side or substrate side of the wafer with an arc lamp or other irradiance device, to heat the wafer at a ramp rate less than 400° C. per second, for example. The subsequent surface heating stage occurs much more rapidly than the thermal conduction time through the wafer, so that only the device side surface is heated to the final annealing temperature, while the bulk of the wafer remains close to the cooler intermediate temperature. Such surface heating may be achieved by exposing the device-side surface to a high-power irradiance flash from a flash lamp or bank of flash lamps, the flash having a relatively short duration, such as one millisecond, for example. The cooler bulk of the wafer then acts as a heat sink to facilitate rapid cooling of the device side surface.
Such annealing methods, which involve rapidly heating the device side of the wafer to a substantially higher temperature than the bulk of the wafer, tend to cause the device side to thermally expand at a greater rate than the rest of the wafer. Depending on the magnitude of the temperature difference between the device side temperature and the temperature of the bulk of the wafer, this may tend to cause “thermal bowing”, whereby the normally planar wafer deforms itself into a thermally deformed shape. Depending on the magnitude and rapidity of the device side heating stage, the thermally deformed shape may have attributes of a dome shape, with the center of the wafer tending to rapidly rise relative to its edge regions. The thermal bowing may also cause the outer perimeter or edge of the workpiece (such as the outer two or four centimeters of a 30-cm diameter wafer, for example) to curl downward steeply, and thus, the thermally deformed shape may also have attributes of a saucer shape similar to a FRISBEE™ flying disc. The thermally deformed shape represents a reduced stress configuration of the wafer, lowering the thermal stress resulting from the temperature gradient between the device side and the bulk of the wafer.
Due to the extreme rapidity at which the device side of the wafer is heated (in the course of a 1-millisecond flash, for example, much faster than a typical thermal conduction time in the wafer), the deformation of the wafer may occur sufficiently rapidly that the edges of the wafer tend to move rapidly downward. If the wafer is supported by conventional support pins near its edges, the thermal bowing of the wafer may apply large downward forces to the support pins, potentially damaging or destroying both the pins and the wafer. Such forces may also cause the wafer to launch itself vertically upward from the support pins, which may result in further damage to the wafer as the wafer falls back down and strikes the pins. If the wafer is supported by support pins located further radially inward, the edges of the wafer may rapidly bow downward and strike a support plate above which the wafer is supported, potentially damaging or destroying the wafer. In addition, due to the rapidity at which such thermal bowing occurs, the initial velocities imparted to the various regions of the wafer tend to cause the wafer to overshoot the equilibrium minimum stress shape and rapidly oscillate or vibrate, resulting in additional stress and potentially damaging or destroying the wafer.